The present invention relates to a linear phase ramp type fiber optic gyro having a looped optical transmission line through which clockwise and counterclockwise light beams are propagated, in which there are generated a ramp signal for providing to the clockwise and counterclockwise light beams a saw-tooth ramp phase of such a slope as to cancel a phase difference between the two light beams produced by an angular velocity inputted to the looped optical transmission line and a biasing signal for alternately giving phase differences of +.pi./2 rad and -.pi./2 rad between the two light beams emitted from the looped optical transmission line whereby the inputted angular velocity is found from
FIG. 1 shows the basic construction of a conventional linear phase ramp type fiber optic gyro disclosed in U.S. Pat. No. 5,031,988, for instance. A light beam from a light source 11 such as a laser is split by beam splitter 12 into two light beams, which enter, as a clockwise light beam 14 and a counterclockwise light beam 15, into a looped optical transmission line 13 formed by, for example, a polarization preserving optical fiber-coil, at opposite ends. The two light beams 14 and 15 having propagated through the optical transmission line 13 in opposite directions and emitted therefrom are coupled together again by the beam splitter 12 and interfere with each other. The resulting interference light is converted by a photoelectric or transducer 16 into an electric signal corresponding to the intensity of the interference light. The AC component of the electric signal is amplified and the DC component of the signal is cut off, by an AC amplifier 17. Interposed between the beam splitter 12 and the two ends of the optical transmission line 13 are first and second optical phase modulators 18 and 19. A biasing signal generator 21 generates a rectangular digital biasing signal V.sub.B of a period 2.tau. and a duty ratio 50% by which the phase difference produced between the light beams 14 and 15 at the time when they have interfered with each other becomes alternately +.pi./2 rad and -.pi./2 rad at intervals of .tau. where t is the duration that the light from the light source 11 takes to propagate through the optical transmission line 13. The biasing signal V.sub.B is applied as a modulation signal to the first optical phase modulator 18. In synchronism with this phase shift and consequently the biasing signal V.sub.B the output of the AC amplifier 17 is synchronously detected by a synchronous detector 22 at intervals of .tau..
The phase difference .PHI. between the clockwise and counterclockwise light beams 14 and 15 when they interfere after being emitted from the optical transmission line 13 and the output I of the AC amplifier 17 bear such a relationship as indicated by the curve 23 in FIG. 2. When the phase difference .PHI. is .+-.2.pi.k rad (k being an integer equal to or greater than 0) the two light beams 14 and 15 intensify each other and the intensity of the interference light is maximum, and when the phase difference .PHI. is .+-.(2k+1).pi. rad they cancel each other and the intensity of the interference light is minimum. When no angular velocity is being input or applied to the optical transmission line 13, the phase difference .PHI. between the clockwise and counterclockwise light beams 14 and 15 alternately changes, with the modulation by the first optical phase modulator 18, positively and negatively by the same value about zero phase as its center of variation at intervals of .tau. as indicated by the curve 24 in FIG. 2. In this instance, the output V.sub.AC of the AC amplifier 17 becomes constant as indicated by the line 25 and the output V.PHI. of the synchronous detector 22 is zero. When an angular velocity is applied to the optical transmission line 13, a phase difference (a Sagnac shift) .PHI..sub.R develops, owing to the Sagnac effect, between the clockwise and counterclockwise light beams 14 and 15 in accordance with the direction and magnitude of the input angular velocity. Under the influence of the phase difference .PHI..sub.R the phase difference .PHI. between the clockwise and counterclockwise light beams 14 and 15 goes positive and negative alternately by the same value about a phase shifted from the zero phase by .PHI..sub.R at its center of variation at intervals of the time .tau. as indicated by the curve 26 in FIG. 2. In consequence, the output V.sub.AC of the AC amplifier 17 alternately goes to positive and negative levels corresponding to the phase difference .PHI..sub.R at .tau. time intervals as indicated by the curve 27, and in this case the output V.sub.AC is either in-phase or 180.degree. out-of-phase with the biasing signal V.sub.B from the biasing signal generator 21. The output level V.PHI. corresponding to the phase difference .PHI..sub.R can be obtained by multiplying the output V.sub.AC of the AC amplifier 17 by +1 and -1, in synchronism with the biasing signal V.sub.B in the synchronous detector 22.
Based on the thus obtained detected output V.PHI. of the synchronous detector 22, a phase cancel signal generator 28 generates a negative feedback signal (a phase cancel ramp signal) V.sub.c which reduces the output V.PHI. to zero. The phase cancel ramp signal V.sub.c has a slope corresponding to the detected level of the synchronous detector 22 and is applied as a modulation signal to the second optical phase modulator 19. The clockwise light beam 14 reaching the beam splitter 12 after being emitted from the optical transmission line 13 is subjected to such a phase shift as indicated by the solid line in FIG. 3A, whereas the counterclockwise light beam 15 is subject to such a phase shift as indicated by the broken line in FIG. 3A with a delay of the propagation time .tau.. Consequently, the phase difference between the two light beams 14 and 15 in the beam splitter 12, caused by the phase modulation by the second optical phase modulator 19, is equal to a phase variation .delta..PHI. per time .tau. on the basis of the phase cancel ramp signal V.sub.C. Since the phase cancel ramp signal V.sub.C cannot be increased infinitely, however, a ramp waveform is used as the phase cancel ramp signal Vc, which repeatedly increases from zero to a shift setting voltage +Vs which provides a phase shift of 2m.pi. (m is an integer equal to or grater than 1). This ramp waveform can easily be generated by producing a reset signal R.sub.p from a reset signal generator 29 to reset the phase cancel signal generator 28 each time when the phase cancel ramp signal V.sub.c reaches the shift setting voltage +Vs as described later. Similarly, when the slope of the phase cancel ramp signal V.sub.C is negative, a ramp waveform is used as the phase cancel ramp signal Vc, which is repeatedly reset to zero each time when the phase cancel ramp signl Vc reaches a shift setting voltage -Vs providing a phase shift -2m.pi.. The overall phase difference between the emitted light beams 14 and 15 in the beam splitter 12, as added to the phase modulation by the biasing signal V.sub.B, is such as shown in FIG. 3C. Hence, by controlling the phase variation .delta..PHI. per time .tau. based on the phase cancel ramp signal V.sub.C so that the output V.PHI. of the synchronous detector 22 is reduced to zero, the phase variation .delta..PHI. becomes equal to the Sagnac phase difference .PHI..sub.R caused by the input angular velocity .OMEGA..
The relationship between the input angular velocity and the Sagnac phase difference .PHI..sub.R is expressed by the following equation: EQU .PHI..sub.R =4.pi.RL.OMEGA./(.lambda.C) (1)
where R is the radius of the optical transmission line 13, L the length of the optical transmission line (i.e. an optical fiber) 13, .lambda. is the wavelength of light emitted from the light source 11, C is the velocity of light in a vacuum and .OMEGA. is the input angular velocity. Then, the input angular velocity is expressed by EQU .OMEGA.=.lambda.C.delta..PHI./(4.pi.RL). (2)
The input angular velocity .OMEGA. and the phase variation .delta..PHI. per time .tau.based on the phase cancel ramp signal V.sub.C bear a linear relationship.
As disclosed in the aforementioned U.S. patent when the phase cancel ramp signal V.sub.C from the phase cancel signal generator 28 has reached the predetermined shift setting voltage .+-.V.sub.s corresponding to .+-.2m.pi. rad, the reset signal generator 29 generates a reset signal for resetting the phase cancel signal generator 28. In other words, the phase cancel ramp signal V.sub.C becomes a sawtooth signal. Hence, given that the time interval between a resetting and the subsequent resetting immediately thereof is T, the next equation (3) is effected EQU .delta..PHI.=2m.pi..tau./T=2m.pi..tau.f (3)
because of the following relationship: EQU T.multidot..delta..PHI./.tau.=2m.tau..multidot.
Substitution of Eq. (3) into Eq. (2) gives EQU .OMEGA.=.lambda.Cm.tau.f/(2RL) (4)
Since .tau.=nL/C (where n is the refractive index of the optical transmission line (i.e. the optical fiber) 13), its substitution into Eq. (4) gives EQU .OMEGA.=.lambda.nmf/(2R) (5)
Thus, the input angular velocity .OMEGA. can be obtained by measuring the frequency f of the phase cancel ramp signal V.sub.C.
When the phase difference provided between the clockwise and counterclockwise light beams 14 and 15 by the phase cancel ramp signal V.sub.C is .delta..PHI., the intensity I of the interference light which is observed in the optoelectric transducer 16 is the light intensity at points A and B in FIG. 4, since the phase difference .delta..PHI. is cancelled by the Sagnac shift phase .PHI..sub.R via the negative feedback loop. The phase difference between the two light beams in the beam splitter 12 when the phase cancel ramp signal V.sub.C is reset is .+-.(2m.pi..+-..pi./2)+.delta..PHI.. Since the term of the phase difference .delta..PHI. is cancelled by the Sagnac shift .PHI..sub.R through the negative feedback loop, the intensity I of the interference light which is observed in the optoelectric transducer 16 becomes equal to the intensity at points where the phase difference is .+-.(2m.pi..+-..pi./2) rad in FIG. 4, for example, at points C and J or D and I when m=1. That is, when the negative feedback loop is stable, the intensity of the interference light observed in the optoelectric transducer 16 is always constant.
In practice, however, it is difficult to make the threshold voltages +V.sub.CP and -V.sub.CN in complete coincidence with the shift setting voltage +V.sub.S and -V.sub.S so that the phase cancel ramp signal V.sub.C is reset when it has reached the shift setting voltages .+-.V.sub.S which provide the phase shifts .+-.2m.pi. rad in the second optical phase modulator 19. Furthermore, since the conversion gain of the second optical phase modulator 19 varies with surrounding conditions such as temperature, the values of the shift setting voltages .+-.V.sub.S. The relative deviation of the positive and negative threshold voltages V.sub.CP and V.sub.CN from the phase shift amounts .+-.2m.pi. in the optical phase modulator 19 is equivalent to the variation of the value m, hence the input angular velocity .OMEGA. cannot correctly be measured, as will be seen from Eq. (5), for example.
Now, consider the case where the phase cancel ramp signal V.sub.C has a positive-going slope as shown in FIG. 5A and the conversion gain of the second optical phase modulator 19 is smaller than the initialized value, that is, the case where the shift setting voltage +V.sub.S necessary for providing the phase shift 2.pi.m is higher than the threshold voltage V.sub.CP and the phase cancel ramp signal V.sub.C is reset before reaching the shift setting voltage V.sub.S (Case 1). In the state where the phase difference .delta..PHI. is cancelled by the Sagnac phase shift .PHI..sub.R' the phase difference .PHI. between the two light beams at the time of resetting is smaller in absolute value than -(2m.pi..+-..pi./2) rad (the case of m=1 being shown) by .DELTA..PHI.. Consequently, the intensity I of the interference light which is observed in the optoelectric transducer 16 at the time of resetting is equal to the intensity at points E and L in FIG. 4, and the output resulting from the synchronous detection of the intensity of the interference light becomes smaller than the synchronous detected output at each of points A and B in FIG. 5A (Case 1).
Now, consider the case where the phase cancel ramp signal V.sub.C has a positive-going slope, the conversion gain of the optical phase modulator 19 is larger than the initialized value and consequently the shift setting voltage +V.sub.S necessary for providing the phase shift 2.pi.m decreases and the threshold voltage V.sub.CP exceeds the shift setting voltage V.sub.S, as shown in FIG. 5B (Case 2). In this instance with the phase difference .delta..PHI. having been cancelled by the Sagnac phase shift .PHI..sub.R' the phase difference .PHI. between the two light beams at the time of resetting is larger in absolute value than -(2m.pi..+-..pi./2) rad by .DELTA..PHI.. Hence the intensity I of the interference light which is observed in the optoelectric transducer 16 is equal to the intensity at points G and N in FIG. 4, and the output V.PHI. resulting from the synchronous detection of the intensity I of the interference light is larger than the synchronously detected output V.PHI. at each of the points A and B.
Now, consider the case where the phase cancel ramp signal V.sub.C has a negative-going slope as shown in FIG. 5C and the conversion gain of the optical phase modulator 19 is smaller than the initialized value (Case 3). In this instance, in the state where the phase difference .delta..PHI. has been cancelled by the Sagnac phase shift amount .PHI..sub.R, the phase difference .PHI. between the two light beams at the time of resetting is smaller in absolute value than +(2m.pi.35 .pi./2) rad by .DELTA..PHI.Consequently, the intensity I of the interference light which is observed in the optoelectric transducer 16 is equal to the intensity at points F and K in FIG. 4 and the synchronous detector output V.PHI. is larger than those at points A and B as shown in FIG. 5C. Consider the case where the phase cancel ramp signal V.sub.C has a negative-going slope as shown in FIG. 5D and the conversion gain of the optical phase modulator 19 is larger than the initialized value (Case 4). In this instance, in the state where the phase difference .delta..PHI. has been cancelled by the Sagnac phase shift amount .PHI..sub.R, the phase difference .PHI. between the two light beams at the time of resetting is larger in absolute value than +(2m.pi..+-..pi./2) rad by .DELTA..PHI.. In consequence, the intensity I of the interference light which is observed in the optoelectric transducer 16 is equal to the intensity at points H and M in FIG. 4, and the synchronous detector output V.PHI. is smaller than those at points A and B as shown in FIG. 5D.
As described above, the shift setting voltages .+-.V.sub.S necessary for providing the phase shifts .+-.2m.pi. fluctuate as the conversion gain of the optical phase modulator 19 varies. A conventional solution to this problem is to correct the positive and negative threshold voltages V.sub.CP and V.sub.CN by means of a threshold correcting circuit 31 shown in FIG. 6, as described below. The output V.PHI. of the synchronous detector 22 is also fed to first and second low-pass filters 32 and 33, the outputs of which are applied to inverting and non-inverting input terminals of a comparator 34, respectively. The cut-off frequency of the first low-pass filter 32 is set higher than the cut-off frequency of the second low-pass filter 33. An up-down counter 35 is controlled by the output of the comparator 34 to count up or down and counts the reset signal (pulses) R.sub.P from the reset signal generator 29. The count value of the up-down counter 35 is applied to a D/A converter 36, which provides a correcting value .delta.V in analog form. The second low-pass filter 33 outputs the mean level of the output of the synchronous detector 22, whereas the first low-pass filter 32 outputs the pulse of the synchronous detector output V.PHI. which is produced at the time of resetting, as depicted in FIGS. 5A to 5D. In Cases 1 and 4, that is, when the output pulse of the synchronous detector 22 at the time of resetting is negative, the output of the comparator 34 goes positive and the up-down counter 35 is put in the count-up state, increasing the correcting value .delta.V. In Cases 2 and 3, that is, when the output pulse of the synchronous detector 22 at the time of resetting is positive, the output of the comparator 34 goes negative and the up-down counter 35 is altered to the count-down state, decreasing the correcting value .delta.V.
The output .PHI.V of the D/A converter 36 is provided to an adder 37, wherein it is added to a reference level V.sub.R from a reference level generator 38, and the added output is applied as the positive threshold voltage V.sub.CP to the reset signal generator 29. Moreover, the output .PHI.V of the D/A converter 36 is also provided to an adder 39, wherein it is added to the output of an inverter 41 which has inverted the polarity of the reference level V.sub.R from the reference level generator 38, and the added output is provided as the negative threshold voltage V.sub.CN. Thus, EQU V.sub.CP =V.sub.R +.delta.V (6) EQU V.sub.CN =-V.sub.R +.delta.V (7)
When the output pulse of the synchronous detector 22 is negative at the time of resetting (Cases 1 and 4), the positive threshold voltage V.sub.CP increases and the absolute value of the negative threshold voltage V.sub.CN decreases. Conversely, when the output pulse of the synchronous detector 22 at the time of resetting is positive (Cases 2 and 3), the voltage V.sub.CP decreases and the absolute value of the voltage V.sub.CN increases.
In this way, when the direction of rotation of the input angular velocity is constant, the threshold voltage of the phase cancel ramp signal V.sub.C is corrected so that it corresponds to the shift setting voltage .+-.V.sub.S at which the phase shift in the second phase modulator 19 is .+-.2m.pi. rad.
FIG. 7 shows another example of the threshold value correcting circuit 31. In this case, the output of the synchronous detector 22 is applied to a differential amplifier 44 as well as to a sample and hold circuit 43 which performs sampling immediately before the biasing signal V.sub.B is switched at .tau. time intervals. The differential amplifier 44 outputs, as an error signal, the difference between the output of the synchronous detector 22 in the current bias phase state (+.pi./2 or -.pi./2) and the output of the synchronous detector 22 in the preceding bias phase state. The error signal from the differential amplifier 44 is applied to an analog integrator 45 and its integrated output is provided as the correcting value .delta.V. In this instance, when the output after resetting of the phase cancel ramp signal V.sub.C is smaller than the output prior to the resetting, the correcting value .delta.V is large, and when the output after the resetting is larger than that before resetting, the correcting value .delta.V is small.
The output correcting value .delta.V of the analog integrator 45 is provided to the adder 37, wherein it is added to the reference level V.sub.R from the reference level generator 38, and the added output is applied as the positive threshold value V.sub.CP to the reset signal generator 29. At the same time, the output correcting value .delta.V of the analog integrator 45 is also provided to the adder 39, wherein it is added to the output of the inverter 41 which has inverted the polarity of the reference level V.sub.R from the reference level generator 38, and the added output is provided as the negative threshold voltage V.sub.CN. Thus, EQU V.sub.CP =V.sub.R +.delta.V (6) EQU V.sub.CN =-V.sub.R +.delta.V (7)
When the synchronous detector output V.PHI. after resetting is smaller than the output before resetting (Cases 1 and 4), the correcting value .delta.V goes positive, and consequently, the absolute value of the voltage V.sub.CP increases and the absolute value of the voltage V.sub.CN decreases. Conversely, when the synchronous detector output .delta.V after resetting is larger than the output before resetting (Cases 2 and 3), the correcting value .delta.V goes negative, and consequently, the absolute value of the voltage V.sub.CP decreases and the absolute value of the voltage V.sub.CN increases.
In this way, when the direction of rotation of the input angular velocity is constant, the threshold voltage at which the phase cancel ramp signal is reset is corrected so that it coincides with the shift set voltage .+-.V.sub.S corresponding to the phase shift amount .+-.2m.pi. rad in the optical phase modulator 19.
As described above, in the conventional linear phase ramp type fiber optic gyro the corrected threshold voltages are expressed by Eqs. (6) and (7), because the threshold value correcting circuit 31 has such a construction as shown in FIG. 6 or 7. For example, in the case where the slope of the phase cancel ramp signal V.sub.C is positive-going (the input angular velocity is clockwise), if the reference level V.sub.R has a deviation .epsilon.V from the shift setting voltage +V.sub.S corresponding to the phase difference +2.pi.m and therefore V.sub.R =V.sub.S +.epsilon.V (where .epsilon.V .gtoreq.0) such as Case 2 (FIG. 5B), then the synchronous detector 22 provides a positive output pulse at the time of resetting. Consequently, the correcting value .delta.V is controlled by the operation of the threshold value correcting circuit 31 to decrease, and when the correcting value .delta.V becomes equal to -.epsilon.V, the negative feedback loop becomes stabilized and the threshold voltage V.sub.CP comes into agreement with the shift setting voltage +V.sub.S. With the circuit constructions depicted in FIGS. 6 and 7, the negative threshold voltage V.sub.CN at this time is determined by Eq. (7), hence ##EQU1## However, since it has been assumed that V.sub.R =V.sub.S +.epsilon.V , its substitution into Eq. (8) gives V.sub.CN =-V.sub.S -2.epsilon.V , which means that the threshold voltage V.sub.CN has a deviation of -2.epsilon.V from the shift set voltage -V.sub.S corresponding to the phase difference -2m.pi.. If the direction of rotation of the input angular velocity is reversed in such a state, the deviation of the threshold voltage V.sub.CN from the shift set voltage V.sub.S is as large as -2.epsilon.V immediately after the reversal of the direction, and hence a measurement error of the input angular velocity is large.
Similarly, when the reference level V.sub.R has the same deviation .epsilon.V as in the above case and the slope of the phase cancel ramp signal V.sub.C is negative (the input angular velocity counterclockwise) (Case 4), the negative feedback loop becomes stabilized when .delta.V becomes equal to +.epsilon.V and the negative threshold voltage V.sub.CN becomes equal to -V.sub.S, but at this time, the positive threshold voltage V.sub.CP obtained from the circuits of FIGS. 6 and 7 becomes V.sub.CP =V.sub.S +2.epsilon.V and it has the deviation 2.epsilon.V from the shift set voltage +V.sub.S. Accordingly, if the direction of rotation of the input angular velocity is reversed in such a state, a large error is caused in the measurement of the angular velocity in the transition period of the negative feedback operation after the reversal of the direction.
To sum up, when the conversion gain of the optical phase modulator 19 is in its steady state, if the direction of rotation of the input angular velocity is reversed and the slope of the phase cancel ramp signal is also reversed, then it is necessary for the correcting value .delta.V to undergo a substantial change from +.epsilon.V to -.epsilon.V or vice versa, resulting in in accurate measurement of the input angular velocity in the transition period.